Glossary

ABSOLUTE PRESSURE
is the arithmetic sum of gauge and atmospheric pressures. It must be
used in all calculations involving the basic gas laws.

ABSOLUTE TEMPERATURE
is the temperature of a body referred to the absolute zero, at which
point the volume of an ideal gas theoretically becomes zero. On the
Fahrenheit scale this is minus 459.67°F; on the Celsius scale it is
minus 273.15°C. Engineering values of minus 460°F and minus 273°C are
used herein.

AFTERCOOLING
involves cooling of gas in a heat exchanger following the completion of
compression to (1) reduce the temperature, and (2) to liquefy
condensable vapors.

ALTITUDE
is the elevation above sea level.

BAROMETRIC
PRESSURE is the absolute
atmospheric pressure existing at the surface of the earth. It is the
weight of a unit column of air above the point of measurement. It varies
with altitude and, at any given location, with moisture content and
weather.

BRAKE HORSEPOWER
is the actual horsepower output to the drive motor.

CAPACITY
of any compressor is the quantity of gas actually delivered when
operating between specified inlet and discharge pressures. For ejectors,
capacity is measured in lb/hr. For all other compressor types, capacity
is a volume measured at the conditions of pressure, temperature, gas
composition, and moisture content existing at the compressor inlet
flange.

CLEARANCE
in a reciprocating compressor cylinder is that volume contained in one
end of the cylinder which is not swept by the movement of the piston. It
includes space between piston and head at the end of the compression
stroke, space under the valves, etc., and is expressed as a percentage
of the piston displacement per stroke. Clearance may be different for
the two ends of a double-acting cylinder. An average is generally used.

COMPRESSIBILITY
is the factor of a gas or a gas mixture that causes it to differ in
volume from that of a perfect gas when each is under the same pressure
and temperature conditions. Occasionally it is called deviation. It must
be determined experimentally.

SUPER-COMPRESSIBILITY is a
term used with various meanings, most frequently the same as
compressibility, although this is not assured. A current ASME Power Test
Code uses it as a ratio of gas densities rather than volumes. Therefore
it is 1/Z in this case. Super-compressibility should never be used
unless its meaning is clarified completely. Compressibility is much to
be preferred and is used herein.

COMPRESSION
EFFICIENCY is the ratio of the
theoretical work requirement (using a stated process) to the actual work
required to be done on the gas for compression and delivery. Expressed
as a percentage, compression efficiency accounts for leakage and fluid
friction losses, and thermodynamic variations from the theoretical
process.

COMPRESSION RATIO
is the ratio of the absolute discharge to the absolute intake pressure.
It usually applies to a single stage of compression, but may be applied
to a complete multistage compressor as well.

DENSITY
is the weight of a given volume of gas, usually expressed in lb/cu ft at
specific temperature and pressure.

DEWPOINT
of a gas is the temperature at which the vapor in a space (at a given
pressure) will start to condense (form dew). Dewpoint of a gas mixture
is the temperature at which the highest boiling point constituent will
start to condense.

DISCHARGE
PRESSURE is the total gas
pressure (static plus velocity) at the discharge flange of the
compressor. Velocity pressure usually is considered only with dynamic
compressors.
Note: Pressure may be expressed as gauge or absolute pressures. psiG
plus atmospheric pressure equals psiA. Note that psiG does not define a
pressure unless the barometric pressure (atmospheric) is also stated.

DISCHARGE
TEMPERATURE is the temperature
existing at the discharge flange of the compressor.
Note: In a multistage compressor, the various stages will have different
discharge pressures and temperatures.

DISPLACEMENT
applies only to positive-displacement compressors. It is the net volume
swept by the moving parts in a unit of time, usually one minute.

DRY-BULB
TEMPERATURE is the ambient gas
temperature.

DRY GAS
is any gas or gas mixture that contains no water vapor and/or in which
all of the constituents are substantially above their respective
saturated vapor pressures at the existing temperature. (See Wet Gas).
Note: In commercial compressor work, a gas may be considered dry (even
though it contains water vapor) if its dewpoint is low at the inlet
condition (say minus 50° to minus 60°F).

DRY UNIT
is one in which there is no liquid injection and/or liquid circulation
for evaporative cooling or sealing.

ENERGY
of a substance is its capacity, either latent or apparent, to exert a
force through assistance, that is to do work.

INTERNAL ENERGY
is that energy which a substance possesses because of the motion and
configuration of its atoms, molecules, and subatomic particles.

KINETIC ENERGY
is the energy a substance possesses by virtue of its velocity of motion.
It enters into dynamic and ejector compressor calculations, but seldom
into positive displacement problems.

POTENTIAL ENERGY
is the energy a substance possesses because of its elevation above the
earth (or above some other chosen datum plane).

ENTHALPY
(Heat Content) is the sum of the internal and external energies.

ENTROPY
is a measure of the unavailability of energy in a substance.

FIXED COMPRESSION
RATIO is the design (built-in)
compression ratio for a rotary unit having this feature.

GAS HORSEPOWER
(See Horsepower)

GAUGE PRESSURE
is pressure as determined by most instruments and gauges. Barometric
pressure must be allowed for to obtain the true or absolute pressure.

GRAVITY
(See Specific Gravity).

HEAT
is energy transferred because of a temperature difference. There is no
transfer of mass.

HORSEPOWER
is a unit of work equal to 33,000 foot-pounds per minute.

HORSEPOWER,
THEORETICAL is the work
theoretically required to compress and deliver a given gas quantity in
accordance with a specified process.

HORSEPOWER,
INDICATED is that obtained by
indicator card analysis of compression or expansion of a cylinder of a
reciprocating compressor. It is the same as gas horsepower.

HORSEPOWER, GAS
is the actual work required to compress and deliver a given gas
quantity, including all thermodynamic, leakage and fluid friction
losses. It does not include mechanical losses.

HORSEPOWER, PEAK
is the maximum power required by a given compressor when operating at a
(1) constant discharge pressure with variable intake pressure, or (2)
constant intake pressure with variable discharge pressure.

HUMIDITY,
in normal usage, has to do with moisture (water vapor) in the
atmosphere. There are two engineering terms involved:

HUMIDITY,
RELATIVE is the ratio of the
actual partial vapor pressure in an air-vapor mixture to the saturated
vapor pressure at the existing dry-bulb temperature mixture.

HUMIDITY,
SPECIFIC is the ratio of the
weight of water vapor in an air-vapor mixture to the weight of dry air.
It is usually expressed as pounds of vapor per pound of dry air.

IDEAL GAS
follows the perfect gas laws without deviation. Practically, there are
no ideal gases, but it is the basis from which calculations are made and
corrections applied.

INDICATED
HORSEPOWER (See Horsepower).

INLET PRESSURE
is the total pressure (static plus velocity) at the inlet flange of the
compressor. Velocity pressure is usually considered only with dynamic
compressors. (See note under Discharge Pressure).

INLET TEMPERATURE
is the temperature at the inlet flange of the compressor.
Note: In a multistage compressor, the various stages may have different
inlet temperatures.

INTERCOOLING
involves the cooling of gas between stages of compression (1) to reduce
the temperature, (2) to reduce the volume to be compressed in the
succeeding stage, (3) to liquefy condensable vapors, and (4) to save
power.

MACH NUMBER
is the ratio of the actual velocity at a given point to the velocity of
sound in the same gas at the conditions existing at this point. These
are known as local conditions.

MECHANICAL
EFFICIENCY is the ratio,
expressed in percent, of the Thermodynamic Work Requirement in the
cylinder to the actual shaft horsepower.

NORMAL AIR
is the term used for average atmospheric air at sea level in a temperate
zone where it contains some moisture. It is defined in the ASME Test
Code For Displacement Compressors as being at 14.696 psiA, 68°F, 36% RH,
and weighing 0.075 lb/cu ft. The K value is 1.395.

PERFECT
INTERCOOLING is obtained when
the gas is cooled to first stage inlet temperature following each stage
of compression.

PERFECT GAS
(See Ideal Gas).

PISTON
DISPLACEMENT of a
reciprocating compressor cylinder is the net volume displaced by the
piston at rated machine speed, generally expressed in cfm. For
single-acting cylinders it is the displacement of the compressing end
only. For double-acting cylinders it is the total of both ends. For
multistage compressors, the displacement of the first stage only is
commonly stated as that of the entire machine.

POLYTROPIC HEAD
is an expression used for dynamic compressors to denote the foot-pounds
of work required per pound of gas.

A
PROCESS occurs whenever the system undergoes either a
change in state or an energy transfer at a steady state (See State).

A
REVERSIBLE PROCESS is an ideal process that may be
stopped and made to retrace its steps and restore to the systems or
surroundings all work and heat previously removed. It is frictionless.

An
IRREVERSIBLE PROCESS is one in which a portion of the
original system energy is dissipated and cannot be returned to the
system through its own operation. The system and/or surroundings cannot
be returned to their original state.

An
ADIABATIC PROCESS is one during which there is no change
in temperature.

An
ISENTROPIC PROCESS is one wherein the entropy remains
constant.

An
ISOTHERMAL PROCESS is one in which there is no change in
temperature.

A
POLYTROPIC PROCESS is one in which changes in gas
characteristics during compression are considered.

PSYCHROMETRY
deals with the properties of air-water vapor mixtures in the atmosphere.

RATIO OF SPECIFIC
HEATS is the ratio of the
specific heat at constant pressure to the specific heat constant volume.
It may vary considerably with pressure and temperature.

SATURATION
occurs when the vapor is at the dewpoint or saturation temperature
corresponding to its partial pressure. A gas is never saturated with a
vapor. The space occupied jointly by the gas and vapor may be saturated,
however.

DEGREE OF
SATURATION is the ratio of the
weight of vapor existing in a given space to the weight that would be
present if the space were saturated at the space temperature.

SATURATED
AIR-VAPOR MIXTURE is one in
which the space occupied by the mixture is saturated with water vapor at
the mixture temperature.

SATURATED VAPOR
PRESSURE is the pressure
existing at a given temperature in a closed vessel containing a liquid
and the vapor from that liquid after equilibrium conditions have been
reached. It is dependent only on temperature and must be determined
experimentally.

SATURATION
PRESSURE is another term for
Saturated Vapor Pressure.

SATURATION
TEMPERATURE is the temperature
corresponding to a given saturated vapor pressure for a given vapor.

SLIP
is the internal leakage within a rotary compressor. It represents gas at
least partially compressed but not delivered. It is determined
experimentally and expressed in CFM to be deducted from the displacement
to obtain capacity.

SLIP RPM
is the speed required of a rotary compressor to maintain a given
discharge pressure, supplying leakage only (zero actual output). The
factor must be established by experiment.

SPECIFIC GRAVITY
is the ratio of the density of a given gas to the density of dry air,
both measured at the same specified conditions of temperature and
pressure, usually 14.696 psiA and 60°F. It should also take into account
any compressibility deviations from a perfect gas.

SPECIFIC HEAT
(Heat Capacity) is the rate of change in enthalpy with temperature. It
is commonly measured at constant pressure or at constant volume. The
values are different and are known as cp and cv respectively.

SPECIFIC HUMIDITY
(See Humidity).

SPECIFIC VOLUME
is the volume of a given weight of gas, usually expressed as cu ft/lb at
specific conditions.

STANDARD PRESSURE
AND TEMPERATURE (SPT) as used
on this web site is 14.696 psiA and 60°F unless specifically stated
otherwise.

STATE
of a system (or part thereof) is its condition at an instant of time as
described or measured by its properties.

SUCTION PRESSURE
is absolute static prevailing at the suction of the ejector.

SUPER-COMPRESSIBILITY (See
Compressibility).

TEMPERATURE
is the property of a substance that gauges the potential or driving
force for the flow of heat. (See also Inlet, Discharge, and Absolute
Temperature).

THEORETICAL
HORSEPOWER (See Horsepower).

A THERMAL
COMPRESSOR is an ejector used
to compress waste or exhaust steam or any other gas through a moderate
range of compression above atmospheric pressure.

VAPOR PRESSURE
is the pressure exerted by a vapor confined within a given space. The
vapor may be the sole occupant of the space, or may be associated with
other gases.

VOLUME
(See Specific Volume).

VOLUMETRIC
EFFICIENCY is the ratio in
percent of the actual delivered capacity (measured at inlet temperature,
pressure and gas composition) to the piston displacement.

WET-BULB
TEMPERATURE is used in
psychrometry and is the temperature recorded by a thermometer whose bulb
has been covered with a wetted wick and whirled on a sling psychrometer.
Taken together with the dry-bulb temperature, it permits determination
of the relative humidity of the atmosphere.

WET GAS
is any gas or gas mixture in which one or more of the constituents is at
its saturated vapor pressure. The constituent at saturation pressure may
or may not be water vapor.

WORK
is energy in transition and is defined in units of Force times Distance.
Work cannot be done unless there is movement.